This invention relates to devices for automatically sensing the home position of a driven element, such as the movable plates of a tuning capacitor. The invention is more particularly directed to a sealed servo tuning capacitor provided for impedance matching purposes with high power RF equipment such as plasma generation equipment. The invention is more particularly directed to a tunable capacitor for an automatic RF matching network to match the impedance of a reactive plasma chamber or similar non-linear load to a constant impedance (e.g., 50 ohms) output of an RF generator or similar RF source.
In a typical RF plasma generator arrangement, a high power RF source produces an RF wave at a preset frequency, i.e., 13.56 MHz, and this is furnished along a power conduit to a plasma chamber. The RF power is also typically provided at a fixed, known impedance, e.g., 50 ohms. Because there is typically a severe impedance mismatch between the RF power source and the plasma chamber, an impedance matching network is interposed between the two. There are non-linearities in the plasma chamber which make it difficult simply to set the impedance match network at fixed positions for a plasma process. For that reason, there are one or more variable tuning capacitors to adjust the line impedance to match the load impedance, and these are controlled by means of error signals, which can be derived from the measured magnitude and phase of the RF wave at the input of the impedance matching network. The phase and magnitude error signals drive servo motors associated with variable tuning capacitors, and drop to a low or zero level when a matched condition has been achieved.
Typically, expensive vacuum capacitors are employed for this purpose, in which the capacitor plates are hermetically sealed within a canister, with a guide shaft that is moved up and down, via a threaded lead screw, with a threaded nut affixed onto the guide shaft to engage the lead screw. In these vacuum capacitors, the movable and fixed plates are fully engaged when the guide shaft and nut are in the fully retracted (down) position, i.e., at the proximal end of travel. The capacitance is reduced as the guide shaft is lifted up, reaching a minimum at the other end of travel.
At the commencement of machine operation, it is crucial for the servo controller to establish the tuning capacitors' home positions, and then move the capacitors, by means of servo motors for example, towards appropriate tuned conditions. Typically, the home position is at one of the end points of travel, e.g., where the fixed and movable capacitor plates are fully engaged (maximum capacitance). At present this is accomplished using electromechanical devices, e.g., microswitches, that turn on to signal the process control microprocessor when the capacitor reaches the end position. An example of a variable capacitor integrating electromechanical end or limit switches is presented in Planta et al. U.S. Pat. No. 5,590,015. In that patent the limit switches are integrated into the capacitor assembly.
A recent proposal to accomplish the home position sensing using fiber optic methods has been to use a pair of fibers, one transmit and one receive. One fiber is fixed in position rigidly to the body of the capacitor, while the other is attached to the movable electrode guide shaft. The fibers are brought into alignment, completing the optical path, as the capacitor is driven into the home position. This proposal has an undesirable lack of any fail-safe capability. That is, the optical circuit is normally OFF. At start up, the stepper motor will drive the capacitor towards its home position, and will continue to drive the capacitor until the optical circuit indicates ON. If for some reason there is a failure in the optical circuit, e.g., a broken or misaligned fiber, or failed photodetector, the capacitor can be driven past the home position and can suffer irreversible damage.
Another recent proposal to accomplish position sensing with optical means employs a first optical fiber that emits light towards the tuning nut and guide shaft of the capacitor, and a second fiber that receives reflected light from the capacitor guide shaft and tuning nut. The semi-reflective surface on the tuning nut and guide shaft allows the circuit to be complete during the normal operating range of travel. The optical path is interrupted at the home position, where the tuning nut passes out of the field of view of the optical fibers, and light is incident on the threads of the lead screw. This method provides the desired normally-closed light path during normal operation, opening upon reaching the home position. The success of this technique depends upon the highly variable light attenuation of the guide shaft being differentiated from the light reflected back from the lead screw at the home position. The electronics necessary in the fiber optic module do not enjoy a cost advantage over the simpler mechanical means of the prior art.
The problems arising from overdriving past the end limit have required secondary mechanical hard limits, for example, an energy absorbing end stop of the type illustrated in Nebiker, Jr. U.S. Pat. No. 4,390,924.